1. Field of the Invention
The present invention relates broadly to mechanisms for feeding, dispensing, delivering, or otherwise handling extremely small or minute items. More particularly, the present invention concerns a feeding mechanism adapted to receive a plurality of randomly-positioned and randomly-oriented extremely small or minute items, such as, for example, capacitors, resistors, or solder preforms, and to isolate, orient, and position one or more of the items in a specific repeatable pickup location wherefrom they may be removed for use by, for example, a computer-controlled automated assembly machine.
2. Description of the Prior Art
It is often desirable to feed, dispense, deliver, or provide extremely small or minute items to a computer-controlled automated assembly machine or other similar equipment, thereby substantially fully-automating an assembly process. In one application, for example, the minute items are surface mount electronic items and the automated assembly machine is operable to automatically incorporate and assemble these items into electronic circuitry. Typically, providing the items to the automated assembly machine is accomplished by a feeding mechanism that positions the items in a repeatable pickup location wherefrom they can thereafter be removed for use by the automated assembly machine. Several different types of feeding mechanisms exist, including, for example, tape reel feeders, waffle-style feeders, and linear or bowl-type vibratory feeders.
Prior art tape reel feeders use items which have been prepackaged in either punched carrier tapes or embossed carrier tapes. The carrier tape is constructed from either paper or plastic and is punched or embossed to form individual pockets for each item. A flat cover tape is adhered to the carrier tape to seal the items in the pockets. In use, the tape reel feeder removes the cover tape from one of the pockets and indexes that pocket forward into the pickup location. Once the exposed item is removed from the pocket by the automated assembly machine, another pocket is uncovered and indexed forward into the pickup location.
Prior art waffle packages involve plastic trays molded into a matrix pattern to form individual pockets for each item, with a single lid covering all of the filled pockets. These packages are typically filled by the vendor. To feed these items, the package is placed on a locating fixture and the lid is removed, whereafter the assembly equipment or machine indexes each pocket in turn and removes the item therefrom.
Prior art vibratory feeders may be of either bowl, linear, or back-and-forth design, each of which uses an electromagnet and a spring to feed the items using vibration. In a bowl design, the items are fed upward along a helical vibratory bowl; in a linear design they are fed along a flat straight line; and in a back-and-forth design they are fed along an inclined straight line. A common variation of vibratory feeding uses items prepackaged in extruded aluminum stick magazines. The magazines are placed on a flat linear feeder and the items are fed forward within the magazines.
These prior art mechanisms are commonly used for feeding or dispensing small items having dimensions of greater than 0.762 mm×0.508 mm×0.1778 mm (0.030 inch×0.020 inch×0.007 inch). It is, however, much more difficult to feed minute items having dimensions of less than 0.762 mm×0.508 mm×0.1778 mm (0.030 inch×0.020 inch×0.007 inch). In prior art tape reels and waffle packages, for example, the size, size tolerance, and positional tolerance of the pockets required to locate the minute items in the repeatable pickup location while maintaining the items' orientation is difficult to achieve. The Electronic Industries Association (EIA) has promulgated Recommended Standard EIA-481-1-90 relating to the dimensions and tolerance of the carrier tape used for tape reel feeding. This standard was written for items larger than 0.762 mm×0.508 mm×0.1778 mm (0.030 inch×0.020 inch×0.007 inch), and recommends that the clearance between the item and the pocket be a minimum of 0.0508 mm (0.002 inch) and a maximum of 0.508 mm (0.020 inch), and that the angle of rotation of the item within the pocket be a maximum of 20°. Given, for example, an item that is 0.381 mm (0.015) inch square, the maximum allowable clearance between the item and the pocket to prevent a rotation greater than 20° is 0.10668 mm (0.0042 inch). For a 0.010 inch square item, the maximum allowable clearance is 0.07112 mm (0.0028 inch).
Where such exact tolerances are not practical, possible, or otherwise met, it is known to use a machine vision system to sense the orientation of the item in a loosely fitting pocket. The machine vision system must be operable to determine the orientation of the item and to compensate for the item's position and degree of rotation. Unfortunately, development and implementation of such a machine vision system is typically exceedingly expensive. Furthermore, operation of the machine vision system, particularly steps involved in processing images, adds undesirably to the total assembly time of the electronic circuit. Additionally, the machine vision system requires extensive programming as well as development of adequate lighting necessary to illuminate the item and the pocket.
Prior art vibratory feeders also suffer from substantial disadvantages and limitations. The forces acting to move the item forward are due to an acceleration of the vibratory feeder and are proportional to the mass of the item. As the item's size decreases, the forces acting to move the item become smaller than the friction, electrostatic, and other forces acting to resist the movement. Furthermore, where the item is pre-packaged in a stick magazine, the fit between the item and the stick magazine must be sufficiently tight to maintain the item in a proper orientation. Again, however, as the item's size decreases, the dimensions, surface finish, and tolerances of the track required to feed the item within the stick magazine and to maintain its proper orientation becomes increasingly difficult to achieve.
Due to the above-identified and other problems and disadvantages in the art, a need exists for an improved mechanism for feeding minute items.